Positive Anti-Scl-70 with Negative ANA: Diagnostic Confirmation and Management
You should repeat the anti-Scl-70 antibody test using immunodiffusion or immunoprecipitation methods, as commercial ELISA-based assays frequently produce false positives—particularly when ANA is negative—and this discordance strongly suggests a false positive result.
Understanding the Diagnostic Dilemma
The combination of negative ANA with positive anti-Scl-70 is highly unusual and should raise immediate suspicion for a false positive result. Anti-Scl-70 antibodies are typically associated with a positive ANA showing a nucleolar or speckled pattern 1, 2. In a case series from a scleroderma referral center, 92.3% of patients who tested positive for anti-Scl-70 by commercial assays but lacked clinical SSc features were actually negative when retested by the more specific immunodiffusion method 1. Notably, 37.5% of these false-positive patients also had negative ANA results 1.
Key Diagnostic Steps
Confirmatory Testing:
- Request anti-Scl-70 testing specifically by immunodiffusion (the most extensively validated method) or immunoprecipitation 3
- These methods have superior specificity (100%) compared to ELISA-based commercial assays 3, 4
- Immunodiffusion remains the gold standard despite being less commonly used in routine practice 2
Clinical Evaluation for Systemic Sclerosis:
Look specifically for these features that would support true SSc:
- Raynaud's phenomenon (absent in 68.8% of false-positive cases) 1
- Skin thickening proximal to the metacarpophalangeal joints
- Nailfold capillaroscopy abnormalities (dilated capillaries, hemorrhages, capillary loss)
- Puffy fingers or sclerodactyly
- Digital ulcers or pitting scars
- Telangiectasias
Apply the 2013 ACR/EULAR classification criteria: patients with false-positive anti-Scl-70 typically have ≤1 clinical criteria items 1.
Managing the Dry Mouth
For xerostomia management, first measure objective salivary gland function before initiating treatment, as subjective symptoms often don't correlate with actual glandular dysfunction 5.
Stepwise Approach to Xerostomia:
Step 1: Measure salivary flow rates
- Whole salivary flow measurement is essential
- Rule out non-SSc causes: candidiasis, burning mouth syndrome, medications 5
Step 2: Treatment based on glandular function:
Mild dysfunction: Start with non-pharmacological stimulation
- Sugar-free acidic candies or lozenges
- Sugar-free chewing gum with xylitol
- These should contain fluoride and have neutral pH 5
Moderate dysfunction: Consider pharmacological stimulation
- Pilocarpine (licensed worldwide for oral dryness)
- Cevimeline (limited availability)
- Note: These have high frequency of adverse events (sweating, urinary frequency) 5
Severe dysfunction: Saliva substitutes
- Oral sprays, gels, or rinses
- Should mimic natural saliva composition with neutral pH and electrolytes 5
Critical Pitfalls to Avoid
Do not pursue extensive organ screening (chest CT, echocardiography) based solely on a positive anti-Scl-70 result without clinical features of SSc. In the false-positive cohort, 43.8% underwent unnecessary chest CT and 29.2% had unnecessary echocardiograms 1.
Do not assume the diagnosis is confirmed without retesting by immunodiffusion. Among patients who retested positive by immunodiffusion, 75% developed classifiable SSc during follow-up, while 0% of those negative by immunodiffusion did so 1.
Recognize that true anti-Scl-70 positivity (when confirmed) predicts diffuse cutaneous involvement, interstitial lung disease, and higher disease severity 3, 2. However, this only applies if the antibody is truly present.
If Confirmatory Testing is Negative
If immunodiffusion confirms negative anti-Scl-70, you can reassure the patient that SSc is highly unlikely. The dry mouth should be evaluated for other causes including Sjögren's syndrome (check anti-Ro/SSA, anti-La/SSB), medication effects, or other salivary gland disorders. Continue symptomatic management of xerostomia as outlined above 5.